7John F H. Factors affecting frozen and fresh embryo transfer pregnancy rates in cattle[J]. Theriogenology, 2001,56 : 401- 415.
8Spell A R, Peal W E, Corab LR, et al . Evaluating recipient and embry of actors that affect pregnancy rates of embryo transfer in beef cattle[J]. Theriogenology,2001,(56) :287-297.
9Donaldson L E . Matching of embryo stages and grades with recipients oestrous synchrony in bovine embryo transfer[J]. VETERINARY RECORD, 1985,19(117) :489-491.